FR2875494A1 - SILICA MICROSPHERES, PROCESS FOR MANUFACTURING, ASSEMBLIES AND POSSIBLE USES OF THESE MICROSPHERES OF SILICA - Google Patents

Abstract

Silica microspheres (M) having an outside diameter between 50 and 125 m, preferably between 60 and 90 m, a wall thickness greater than 1 m, preferably between 1 and 3 m and a density between 0, 3 and 0.7g/cm3, manufacturing process by injection of silica microsphere precursors (MS, PR1, PR1', PR2') within an inductive plasma (P), assemblies and possible uses of silica microspheres ( M).

Description

2875494 1

  The present invention relates to silica microspheres, their method of manufacture, assembly and various possible applications of silica microspheres.

  Patent FR 2 619 101 discloses a process for producing silica microspheres whose composition shows a high silica content. Such a process involves two distinct steps: the heat treatment of crushed glass particles by the use of a conventional burner followed by a chemical treatment of dealkalizing the particles. By this method, are obtained, following the two treatments, silica microspheres whose silica weight composition is relatively high. This process has particular interest in the recycling of cullet. However, the use of a conventional burner has several disadvantages. First of all, according to this document, the use of a conventional burner does not, on its own, make it possible to obtain a very high silica purity level, which requires a second treatment, of a chemical nature, of which the implementation is delicate.

  In addition, the use of a conventional burner allows the heat treatment of particles at a temperature of 1500 C to 1750 C. Therefore, the treatment of a base material having a high silica purity level and which comprises thus a higher melting temperature is therefore very difficult by this method.

  Finally, the use of this process makes the control of the temperature and the balance of energy and material unsatisfactory, which has the possible consequences of energy loss, a yield of reduced silica microspheres, the obtaining of a greater amount of silica nanoparticles (silica soot) and inhomogeneity of particle size.

  The object of the invention is therefore to provide microspheres of silica silica of high purity in silica, of regular shape and size, and a process for producing these silica microspheres which avoids the aforementioned drawbacks.

  For this purpose, the subject of the invention is a silica microsphere having an outside diameter of between 50 and 125 m, preferably of between 60 and 90 m, a wall thickness of greater than 1 m, preferably of between 1 and 3. m and a density of between 0.3 and 0.7 g / cm 3. Thus, because of their low density and the thinness of their walls, the silica microspheres form a very light material and can therefore be used as a material for lightening the structure in which the silica microspheres are integrated (for example by inclusion in concrete). However, the walls of the silica microspheres are sufficiently thick to give the silica microspheres satisfactory mechanical strength and in particular a high compressive strength which is greater than MPa / m 2.

  Preferably, the silica microsphere comprises more than 95% by weight of silica relative to the total mass of the silica microsphere, preferably more than 99% by weight of silica relative to the total mass of the silica microsphere.

  Therefore, silica microspheres having a purity level greater than 95% (defined by the mass of silica relative to the total mass of the silica microsphere) can be used in high temperature applications (greater than 1600 C) .

  In addition, the silica microspheres according to the invention have a large specific surface area of 0.08 to 0.1 m 2 / g which enables them to have a large contact surface with the surrounding medium.

  Finally, the silica microspheres have an amorphous structure (because the structure is vitrified during the heat treatment) which allows them to be recognized as non-carcinogenic and non-pulverulent, which allows them to be handled safely by the user.

  The subject of the invention is also a process for the production of silica microspheres according to the invention, characterized in that it comprises at least one step of injecting at least one precursor of the silica microspheres into a inductive plasma, said inductive plasma being preferentially doped with a hydrocarbon such as propane or methane. The term precursors of silica microspheres should be interpreted broadly. Indeed, the precursors of silica microspheres that are injected into the inductive plasma may be of different types: silica glass powder and / or unpretreated quartz, silica microspheres, or even glass powder of silica and / or quartz or pretreated sections of silica microtubes. Different types of precursors can be injected into the inductive plasma. Advantageously, only one type of precursor is injected into the inductive plasma.

  The term within an inductive plasma can be interpreted as in an inductive plasma reactor, in an environment close to the inductive plasma or, where appropriate, in the center of an inductive plasma.

  This process allows the formation of hollow silica microspheres having a very high silica purity level greater than 95%, preferably greater than 99%. The silica purity level of the silica microspheres obtained by the process according to the invention, which depends on the silica purity level of the precursor, is higher than the silica purity level of the precursor (except when the precursor, in the form of silica powder or quartz, microsphere or sections of silica microtubes, has a very high silica purity level that is to say greater than 99.99%).

  This process can thus, for example, be applied to the recycling of silica microspheres. In this case, the precursor consists of silica microspheres which it is desirable to increase the degree of purity, the precursor may consist of silica microspheres from the process according to the invention or any other method.

  This process also makes it possible to heat-treat materials having a silica purity level greater than 99.99%.

  Finally, this process allows good control of the various reaction parameters such as temperature, energy and material fluxes. Thanks to this process, it is possible to obtain silica microspheres with a more uniform particle size distribution than with a conventional burner type heat treatment.

  Advantageously, the method may further comprise a silicon halide injection step. Preferably, the silicon halide injection step is simultaneous with the injection of the precursor. This step makes it possible to further increase the silica purity level between the precursor of the silica microspheres and the silica microspheres. This step makes it possible to boost the microspheres to halogen, increasing its properties according to the applications. In addition, this step is simultaneous with the inductive plasma treatment, which represents a time saving for the process.

  According to a preferred embodiment of the method, the method further comprises a step of pretreating the precursor of the silica microspheres prior to the step of injecting said precursor into an inductive plasma. This pretreatment can be of the wet or dry type.

  According to a first variant, the precursor of the silica microspheres is a silica or quartz glass powder having a particle size of less than 5 m, preferably less than 211 m.

  The pretreatment is preferably carried out wet, that is to say in the presence of water, for this first type of precursor. The use of silica glass or quartz powder having the proposed particle size allows obtaining silica microspheres with a uniform particle size distribution, wall thickness and density.

  Preferably, according to this first variant, the pretreatment step of the precursor of the silica microspheres consists in mixing the precursor of the silica microspheres in water with at least one additive which is an expansion agent and / or a binding agent and / or a fluxing agent, the precursor composition resulting from the mixture being then injected into an inductive plasma. The water is preferably demineralised water. This pretreatment step of the precursor of silica microspheres, wet, allows to further increase the homogeneity of the particle size distribution. This step also confers greater homogeneity of the other structural parameters of the silica microspheres such as wall thickness and density.

  Advantageously, the precursor composition comprises at least 50% by weight of precursor of the silica microspheres relative to the total mass of said precursor composition, preferably at least 60% by weight of silica microsphere precursor relative to the mass. total of the precursor composition.

  Preferably, the precursor composition comprises precursor silica microspheres and seawater. This variant preparation of the precursor composition has the advantage of being less expensive.

  According to a first embodiment of said first variant, the precursor composition is directly injected into an inductive plasma.

  According to a second embodiment of said first variant, the precursor composition is further concentrated by filtration before being injected into an inductive plasma. This step makes it possible to increase the silica content of the precursor composition before injection within the inductive plasma in order to increase the silica purity level of the silica microspheres.

  Advantageously, the filtration step further comprises a granulometric separation step before injection. This step makes it possible, by reducing the particle size distribution of the precursors of silica microspheres, to obtain silica microspheres with a narrower particle size distribution.

  According to a third embodiment of said first variant, droplets of said precursor composition are coated with synthetic silica powder before being injected into an inductive plasma. This step makes it possible to increase the silica content of the precursor composition before it is injected into the inductive plasma in order to increase the silica purity level of the silica microspheres.

  Preferably, the step of coating with synthetic silica powder is carried out by spraying the precursor composition on a vibrating table supporting an anti-wetting agent and said synthetic silica powder, the anti-wetting agent being preferentially constituted plant material, in particular lycopodium spores. The anti-wetting agent may also consist of any other material of plant origin and in particular fern spores.

  According to a second variant of the preferred embodiment of the invention, the precursor of the silica microspheres is in the form of silica microtubes. This type of silica microsphere precursor requires a pretreatment step, dry, prior to injection into the inductive plasma.

  Advantageously, the silica purity level of the silica microtubes is greater than 99.9% by weight. The silica purity level of the silica microtubes is preferably greater than 99.99% by weight.

  Preferably, the pretreatment step of the precursor of the silica microspheres comprises a step of cutting the laser silica microtubes. This step of cutting the laser silica microtubes makes it possible to obtain sections of microtubes directly injectable within the inductive plasma. These sections have, after cutting by a laser, welded ends.

  Advantageously, before the cutting step, the microtubes are etched at their upper generatrix chemically, thermally or mechanically. This grinding step saves part of the energy consumed by the laser during the priming of the cut at the level of the upper generatrix of the microtubes.

  Preferably, the step of cutting the silica microtubes further comprises a simultaneous scanning step with a flushing gas. The flushing gas is preferably hydrogen or helium. The sweep gas is thus trapped in the sections whose ends are welded by laser cutting.

  The invention also relates to the assembly of silica microspheres.

  According to a first variant, the assembly of the silica microspheres is carried out by agglomeration of the microspheres with an agglomerating agent. The agglomerating agent consists of any substance that can coat and bond the silica microspheres together. The agglomerating agent is, for example, a synthetic or natural polymer and preferentially starch. The assembly according to said first variant makes it possible to obtain lightweight materials that can be produced in multiple forms in a simple and inexpensive manner that can notably be used as thermal insulation material in the field of the building.

  According to another variant, the assembly of the silica microspheres is carried out by batch sintering which comprises the steps of: Mixing microspheres of silica and silica nanopowder, Packing the composition resulting from this mixture into a mold, Preheat the composition by applying microwaves preferably in the presence of oxalic acid vapors and heat the preheated composition in a thermal oven.

  The nanopowder (or so-called submicron powder) of silica consists of silica particles having a particle size of between 250 and 1000 nm and is a by-product of the thermal treatment of silica.

  According to a last variant of application, the assembly of the silica microspheres is carried out by continuous sintering, which comprises the steps of: Mixing microspheres of silica and nanopowder of silica, heating the composition resulting from this mixing in the presence of an electrical resistance by depositing said composition continuously on a treadmill and temporarily immobilizing said composition at the electrical resistance.

  The sintering technique provides materials that can withstand temperatures up to 1650 C or even 2400 C when the assembled material is used for wear parts.

  The invention also relates to the use of silica microspheres.

  According to a first variant, the silica microspheres are used in the flocking of surfaces such as concrete, plaster or a metal alloy.

  Preferably, the flocking comprises a first step of mixing the silica microspheres with silica nanopowder and silica fibers and a second step of projecting the composition resulting from said mixture of the first step by thermal lance. The flocked surface can withstand very high temperatures. This type of flocking can be envisaged for applications in the fields of aerospace, shipbuilding and fire protection in the building and the construction of infrastructure equipment for example.

  According to a second variant, the silica microspheres are used as thermal insulation material.

  According to a third variant, the silica microspheres are used for storing gas. Preferably, the silica microspheres are used for the storage of gases such as helium or hydrogen.

  Advantageously, the gas storage method comprises a step of heating silica microspheres at a temperature greater than or equal to 800 ° C. in the presence of helium or of hydrogen at a pressure greater than or equal to 10 Pa.

  Preferably, a step of scanning with helium is provided prior to the heating step under pressure.

  Advantageously, a progressive cooling step under a pressure greater than or equal to 10 Pa is provided consecutively to the heating step under pressure.

  The gas stored in the silica microspheres is preferably released by a heating step at a temperature above 850 C at atmospheric pressure.

  The invention will be better understood, and other objects, details, features and advantages thereof will appear more clearly in the following detailed explanatory description of an embodiment of the invention given as a purely illustrative and non-limiting example, with reference to the accompanying schematic drawings.

  In these drawings: FIG. 1 is a diagrammatic sectional representation of a mixer allowing the preparation and homogenization of an aqueous composition comprising the precursor of the silica microspheres, this step being carried out during pre-treatment by wet prior to step of injecting the precursor into the inductive plasma; Figure 2 is a schematic cross-sectional representation of the distribution of the retarders in the slowing chamber around the mixer tank; Figure 3 is a schematic representation of a motor sprayer for withdrawing the composition of the mixer; Figure 4 is a schematic sectional representation of a vacuum injector for withdrawing the composition of the mixer; Figure 5 is a schematic sectional representation of a device for filtering the composition from the mixer; FIG. 6 is a schematic perspective representation of a granulometric separation device comprising a single sieve; FIG. 7 is a diagrammatic representation in side section of a granulometric separation device comprising two sieves; FIG. 8 is a diagrammatic cross-sectional representation of a horizontal drum mixer for mixing the anti-wetting agent and the synthetic silica powder; FIG. 9 is a diagrammatic sectional representation of a cylindrical box mixer for mixing the anti-wetting agent and the synthetic silica powder; Fig. 10 is a schematic side view of an inclined vibrating table for embedding droplets of the precursor composition with a mixture of the anti-wetting agent and the synthetic silica powder; Figure 11 is a schematic representation in longitudinal section of an inductive plasma reactor; Figure 12 is a schematic sectional representation of a sintering preparation device; Figure 13 is a schematic sectional representation of a discontinuous sintering device; Figure 14 is a schematic sectional representation of a continuous sintering device; FIG. 15 is a diagrammatic representation in perspective of a laser cutting device of silica microtubes, and FIG. 16 is a schematic representation in section of a reactor allowing the filling of silica microspheres with gases such as helium or hydrogen.

  In the detailed description which follows, the terms precursor (s) of silica microsphere (s), silica microsphere (s) and silica microtube (s) will respectively be designated by the terms precursor (s), microsphere (s) and microtube (s). In addition, the term within an inductive plasma can be interpreted as in an inductive plasma reactor, in an environment close to the inductive plasma or, where appropriate, in the center of an inductive plasma. Finally, the term injected, applied to the feed of the plasma reactor precursor means projected when the precursor is in the form of a slurry of silica powder or quartz, or sections of silica microtubes or microspheres of silica and pulverized when the precursor is in the form of either a light suspension of silica powder or silica powder or quartz.

  The heat treatment of the precursors by inductive plasma is for example carried out in a plasma reactor 1 shown in FIG.

  The reactor 1 is of generally cylindrical shape having ends la and lb closed.

  The reactor 1 has at its center, in the longitudinal axis A of the reactor 1, a torch 2 with inductive plasma P. The torch 2 has a body 2a of generally cylindrical shape with a circular section having ends 2b and 2d open. The body 2a of the torch 2 is composed, in a radially external direction, of a succession of silica tubes (not shown) whose axis of rotation passes through the axis A, which have an increasing diameter and whose tube larger diameter is surrounded by a steel sleeve having a cooling system by passage of demineralized water (not shown). The body 2a of the torch 2 is surrounded by an inductor 7, which is a Teflon-coated copper coil, cooled with demineralised water, connected to the terminals of a high-voltage industrial high-frequency generator (not shown). (of the order of 10kV), frequency between 2 and 4 MHz and power between 50 and 500 kW, preferably between 90 and 200 kW.

  The body 2a of the torch 2 has at its center the plasma P which is schematized by an ellipse and which extends along the longitudinal axis A. The plasma P has a height greater than that of the body 2a of the torch 2 and protrudes from the body 2a of the torch 2 at the upper end 2b of the torch 2. The inner wall of the body 2a is preferably covered with a slip (not shown) to increase the life of the torch 2.

  The side wall 1c of the reactor 1 is traversed by different ducts: the precursor supply duct 3 which may be a rod or an injection or spray torch, the supply duct 4 into the plasma gas G, and the duct Silicon halide feed line H. The plasma gas feed duct 4 also passes through the side wall of the body 2a of the torch 2 at its lower end 2d.

  The precursor feed duct 3, advantageously constituted by silica, conveys the precursor MS, PR1, PR1 'and PR2' preferentially via an injection gas (not referenced) which may be argon, helium and preferably compressed dry air. The injection gas is preferably of the same nature as the plasma gas G which may be argon, helium and preferably dry air. The plasma gas G may be advantageously doped by the introduction of hydrogen, or a hydrocarbon fuel, such as methane or propane to enhance the thermal conductivity of the plasma.

  In addition, an evacuation device 6 of the microspheres M, schematized in the figure by a trapezium, passes through the wall of the reactor 1 at the junction between the end wall la and the side wall 1c. The evacuation device 6 of the microspheres M may be a recovery cyclone discharging the microspheres M, by suction, in the direction illustrated by the reference arrow V. Finally, an evaporator 8 of the silicon halide H, shown schematically by a hexagon, communicates with the silicon halide feed line H. When energizing the inductor 7, an alternating magnetic field is generated at the center of the body 2a of the torch 2. The plasma gas G can be dry air, argon or helium is conveyed by the supply duct 4 to the center of the body 2a of the torch 2 and then, in the presence of the magnetic field induced by the inductor 7, undergoes an elevation temperature (up to 10000K when the plasma gas G is dry air). The plasma P is thus initiated and then maintained.

  The precursor may be a silica or quartz powder having a particle size of less than 5 m having undergone previous pretreatment wet (precursor PR1 ') or not (precursor PRO). The precursor may also be in the form of sections (precursor PR2 ') or in the form of silica microspheres (precursor MS).

  The precursor PR2 'advantageously has a silica purity level of 99.99%. The other types of precursors PR1, PR1 ', MS exhibit, according to the silica purity level desired for the silica M microspheres, a variable silica purity level: greater than 99.0%.

  The precursor MS, PR1, PR1 'or PR2' conveyed by the injection gas is injected through the precursor feed pipe 3, at the upper end of the torch 2 into the plasma P. When the precursor is a pretreated silica or quartz powder PRI 'or non PR1 or microspheres silica MS, its injection into the reactor 1 causes two phenomena: the volume expansion due to the vaporization of water (especially in the case of precursor PR1 ') and the release of CO and CO2 by the organic impurities (especially in the case of the precursors MS, PR1 and PR1'), followed by the surface melting of the precursor particles (especially in the case of precursors PR1 or PR1 '). The volume expansion results in the creation of a hollow internal zone and a decrease in the density of the microspheres M. The surface melting generates, due to the surface tension, the formation of the spheroidal structure of the microspheres M. When the precursor is in the form of sections PR2 'of silica microtubes PR2, its injection into the reactor 1 causes above all the surface melting phenomenon of the precursor particles PR2' which also results in the formation of the spheroidal structure of the microspheres M. Advantageously, a device for rotating the precursors PR2 'is used (for example by creating a rotational movement within the injection gas in the precursor feed duct 3).

  A drop in the reaction temperature can occur because of the energy consumed by the vaporization of water and impurities.

  This drop in temperature can be compensated either by injecting, during the reaction, at the supply duct 4 with plasma gas G, fuel gases such as methane or propane, or by adding a plasma gas G, hydrogen or helium, either by increasing the heat generated by the plasma P by increasing the electric power within the inductor 7.

  In addition, silicon halide H can be fed through the feed duct 5 to the center of reactor 1, simultaneously with the transport of precursor PR1, PR1 'or PR2'. The silicon halide H is previously vaporized at 95 ° C. by an evaporator 8 to 10 double chamber for example, so as not to induce a drop in the internal temperature of the reactor 1. The silicon halide H, which may be a halide at chlorine, fluorine, iodine or bromine, and preferably a silicon tetrachloride, has the role of increasing the silica content of the microspheres M in two ways: by the addition of synthetic silica which is associated with the surface of the precursor particles PR1, PR1 'and PR2', and by the removal of metallic impurities (boron oxides and alkaline oxides) associated with the precursor particles PR1, PR1 'by the reaction of these metal impurities with the halogen atoms of the silicon halide H, and their volatilization at a temperature different from the silicon oxide.

  When the microspheres M are formed, they are evacuated by suction by the evacuation device 6 of the microspheres M which may be a recovery cyclone.

  As stated above, a new heat treatment step within the inductive plasma reactor P can be carried out from the MS microspheres (from the process or any other process for producing silica microspheres) to enhance the stability of the walls. or further purifying the silica contained in the MS microspheres (recycling process).

  The heat treatment can also be performed in inductively coupled plasma reactors of different configuration, for example having three to four torches which can be arranged for example vertically down the reactor, laterally or still inclined up or down the reactor . The torches may have an internal diameter of 50 to 100 mm. In addition, the precursor supply ducts 3 can be placed in the center of the body 2a of the torch 2, at the axis A. Finally, a plurality of supply ducts 3 (two to six supply ducts 3) can be arranged outside the torch 2 in the direction of the outlet of the plasma P. In order to obtain a better homogeneity of the structural characteristics of the microspheres M such as the particle size, the wall thickness and the density, from precursor PR1, pretreatment is performed prior to injection. This pretreatment, carried out wet is illustrated in Figures 1 to 9.

  This prior wet pretreatment comprises the preparation of a PR1 precursor which is in the form of an aqueous PR1 'precursor composition.

  The preparation and homogenization of the precursor composition PR1 'are carried out in a mixer 10 as shown schematically in FIG.

  The mixer 10 is in the form of a vessel with a volume preferably of the order of 1m3, consisting of two generally cylindrical walls and having the same longitudinal axis B: the outer wall 11 and the inner wall 12 whose diameter is less than that of the outer wall 11.

  The space defined within the inner wall 12 is a homogenization chamber 13 and the space defined between the inner wall 12 and the outer wall 11 is a slowing chamber 14.

  The mixer 10 is provided with a homogenizer 16 consisting of a homogenizing blade 16a, a rotation shaft 16b and a motor 16c. The homogenizing blade 16a and the lower part of a rotation shaft 16b are arranged in the center of the homogenization chamber 13, at the level of the axis B. A feed rod 17a PR1 precursors, a cane d additive AD (which may be a blowing agent and / or a binding agent and / or a fluxing agent) and a feed rod 17c in demineralized water W (preferably with a resistivity greater than or equal to 1 Mn.cm) have a funnel-shaped upper part which extends in their lower part in a duct immersed in the homogenization chamber 13. The lower part of the feed rod 17a PR1 precursors and that of the supply rod 17b additive AD extend to the level of the homogenizing blade 16a. The lower part of the feed rod 17c in demineralized water W extends only in the upper part of the homogenization chamber 13. The additives AD are, for example, ammonium, calcium and sodium compounds of the chloride type. sodium or ammonium nitrate.

  The retarders 12a, 12b, 12c, 12d, diagrammatically illustrated in FIGS. 1 and 2, are in the form of parallelepipeds parallel to the axis B and are arranged in the deceleration chamber 14. Each pair of retarders 12a, 12b and 12c 12d consists of two retarders 12a and 12b or 12c and 12d arranged against the outer surface of the inner wall 12 along their longer edge. The retarders of the same pair of retarders 12a and 12b or 12c and 12d are arranged in the same plane, on either side of the homogenization chamber 13 symmetrically with respect to the axis B, and each pair 12a, 12b and 12c, 12d is disposed in perpendicular planes intersecting along the axis B as clearly visible in Figure 2. Alternatively, the assembly may have more than two pairs of retarders if necessary.

  A withdrawal rod 18 consists of a duct whose lower end dips into the bottom of the slowing chamber 14 and whose upper end curves outwardly of the mixer 10.

  During the wet pretreatment stage, the precursor PR1, the additive AD (blowing agent and / or binding agent and / or melting agent) and the demineralized water W are respectively conveyed continuously by the canes d supply 17a, 17b, 17c within the homogenization chamber 13 and are mixed and homogenized by the homogenizing blade 16a whose rotation movement generated by the motor 16c, is transmitted by the rotation shaft 16b. The precursor composition PR1 'resulting from the mixture is transferred into a slowing chamber 14 by overflow of the homogenization chamber 13.

  Then the precursor composition PR1 'is withdrawn by the withdrawal rod 18 by pumping carried out by a motorized sprayer 19a or a vacuum injector 19b with the injection of a driving fluid F respectively diagrammatically illustrated in FIGS. 4.

  The precursor composition PR1 'is either conveyed to the precursor feed duct 3 for feeding directly to the plasma reactor 1, or conveyed to a concentration device by filtration or to a concentration device by coating with silica.

  Advantageously, the precursor composition PR1 'comprises at least 50%, and preferably 60%, by mass of precursor of the silica microspheres PR1' with respect to the total mass of said precursor composition PR1 '.

  Preferably, the precursor composition PR1 'comprises about 10 parts per million by weight of additives AD relative to the total mass of said precursor composition PR1'.

  According to another variant, the wet pretreatment can be carried out by mixing the precursor PR1 with only seawater, which makes it possible to avoid the use of additives AD and demineralised water W. This variant of process leads to a simplification of the process and a saving relative to the purchase of AD additives and the use of demineralized water W. Moreover, this process variant makes it possible to obtain a silica purity in the microspheres M equivalent or more important that when the process uses additives AD because AD additive residues can be found in the microspheres M. The precursor compound of seawater must however be injected at more than 50mm of the turns of the inductor or should preferably be previously treated so as to reduce its electrical conductivity.

  FIG. 5 illustrates the filtration concentration device 20 which consists, in its upper part, of a filter screen 21 and in its lower part, of a funnel-shaped receptacle 22 which ends in its lower part. in a vent pipe 27.

  The filter screen 21 consists of a base 23 of circular section and perforated with holes 10 to 15 m in diameter, a cylindrical wall 24 and a textile filter 25 disposed over the entire surface of the base 23 in the space defined by the cylindrical wall 24.

  The precursor composition PR1 'is deposited via the hopper 29 on the surface of the textile filter 25. By application of a depression at the receptacle 22 or by gravity, the excess water 28 in the precursor composition PR1 ', that is to say the water not bound to the precursor particles PR1', which may be accompanied by particles of particle size less than 10 to 15 m (not shown), passes successively through the textile filter 25 and the base 23 and collected in the receptacle 22 before being discharged through the discharge conduit 27. The collected water 28 can be discarded or used to replenish the mixer 10 by connecting the discharge conduit 27 to the supply rod 17c. The concentrated PR1 'precursor composition is then in the form of a friable filter cake 26. The filter cake 26 is then removed from the filter screen 21 by means of the textile filter 25 which supports it. It is then preferentially broken (step not shown) in order to disaggregate the particles of the precursor composition PR1 '. Then the broken filter cake 26 can directly feed the precursor supply duct 3 in order to be injected into the inductive plasma reactor 1 P or be subjected to a granulometric separation step.

  In FIG. 6, the granulometric separation device 30a comprises a granulometric separation screen 31 (of the Rhevum trademark for example) which has meshs of approximately 70 m (not shown), a feed hopper 32 and vibrating hammers. 33. The vibrating hammers 33 are located against the lateral parts of the lower face of the granulometric separation screen 31.

  The granulometric separation screen 31 which is inclined and adjustable in inclination, has an upper portion 31a and a lower portion 3 lb. In FIG. 7, the granulometric separation device 30b comprises an additional granulometric separation screen 34 arranged parallel to and below the first granulometric separation sieve 31. The second granulometric separation sieve 34 has meshs of approximately 50 m (not shown). ). This figure also shows the receptacles 37, 36 and 35 in the form of a funnel. The vibrating hammers 33 are, for their part, not shown in Figure 7.

  The broken filter cake 26 is placed in the feed hopper 32 which deposits it at the top 3a1 of the granulometric separation sieve 31. The vibrating hammers 33 transfer back and forth to the sieve separation sieve 31 in both the axial, longitudinal and transverse directions. The particles of the precursor composition PR1 'with a particle size greater than 70 m 37' roll by gravity from the upper part 31a to the lower part 3b. As illustrated in FIG. 7, the particles of the precursor composition PR1 'with a particle size of less than 70 m 35' and 36 'pass through the mesh of the granulometric separation sieve 31. The particles of the precursor composition PR1' of particle size less than 70 m 35 'and 36' are then subjected to a second granulometric separation step. Only particles having a particle size of less than 50 m pass through the size separation sieve 34 and are collected in the receptacle 35. The receptacles 37, 36 and 35 respectively collect the particles of the precursor composition PR1 'with a particle size greater than 70 m 37 ', between 50 m and 70 m 36', and less than 50 m 35 '. Then particles of selected particle size 35 ', 36' or 37 'are directed to the precursor feed duct 3 to be injected into the inductive plasma reactor 1.

  The granulometric separation step can also take place before injection of a precursor MS or PR1 which has not been subjected to any pretreatment. However, a separation screen having different size mesh is used when the PR1 precursor is subjected to a size separation step.

  This step makes it possible, by reducing the particle size distribution of the precursors of silica microspheres, to obtain silica microspheres with a narrower particle size distribution. It is obvious any particle size separation device having structural variants such as a number of sieves or a different mesh size can be used in the context of the present invention.

  According to another variant illustrated in FIGS. 8 to 10, the precursor composition PR1 'is concentrated by coating with a composition L resulting from the mixture of synthetic silica powder S and an anti-wetting agent AM before being injected with FIGS. 8 and 9 illustrate mixing devices 40 and 50 of the anti-wetting agent AM and the synthetic silica powder S. The anti-wetting agent AM can for example consist of plant spores and in particular lycopodium spores.

  In FIG. 8, the propeller-type mixing device 40 comprises a cylindrical tank 41 having in its center an agitator 42 consisting of a double-blade propeller 42a placed in the longitudinal axis C of the mixing device 40 and connected to a motor (not shown) by a rotation shaft 42b. The diameter of the helix 42a extends over approximately two thirds of the diameter of the cylindrical tank 41.

  Two hoppers represented by the references 43 and 44 and which have a generally funnel shape respectively supply the mixing device 40 with anti-wetting agent AM and with synthetic silica powder S. The propeller 42 a mixes and homogenizes dynamically the composition L resulting from the mixture of synthetic silica powder S and an anti-jamming agent AM. A receptacle 45 collects the composition L. In FIG. 9, the static baffle type mixing device 50 comprises a cylindrical vessel 51 having, along the entire internal surface of the wall of the cylindrical vessel 51, straight baffles 52 arranged obliquely. with respect to the longitudinal axis D of the mixing device 50.

  Two hoppers represented by the references 53 and 54 which have a generally funnel shape respectively feed the mixing device 50 AM anti-wetting agent and synthetic silica powder S. The passage between the straight baffles 52 mixes and homogenizes static composition L from the mixture of synthetic silica powder S and an anti-wetting agent AM. A receptacle 55 collects the composition L. The anti-wetting agent AM is advantageously constituted by lycopodium spores and is preferably present in the composition L in a weight ratio of 20% relative to the total weight of the composition L. Mass ratio may vary within a range for which the resulting composition L remains physicochemically stable.

  FIG. 10 illustrates the coating step which is carried out by spraying the precursor composition PR1 'on a vibrating table 60 supporting the composition L. The vibrating table 60 is inclined thus defining an upper part 60a and a lower part 60b. The means transmitting the vibrations to the vibrating table 60 are not represented. The vibrating table 60 is made of stainless steel and is preferably covered with Teflon.

  A hopper represented by the reference 61, connected to the open bottom portions of the receptacles 45 and 55 of FIGS. 8 and 9 respectively and represented in the form of a cylinder, deposits the composition L at the top 60a of the vibrating table 60. from the mixture of synthetic silica powder S and an anti-jamming agent AM. The composition L extends from the upper part 60a towards the lower part 60b and towards the lateral edges of the vibrating table 60 by the combined action of gravity and vibrations of the vibrating table 60.

  An injection rod 64 sprays droplets 65 of the precursor composition PR1 'at the lower middle portion 60c of the vibrating table 60. The droplets 65 roll by the combined action of the vibrations of the vibrating table 60 and the gravity towards the lower part 60b of the vibrating table 60 surrounding the composition L to form coated droplets 66.

  The anti-wetting agent AM is an agent which allows the droplets of precursor composition PR1 'during their projection on the synthetic silica S, not to impregnate the synthetic silica S but to roll over the synthetic silica S and coat them.

  The coated droplets 66 are collected by a receptacle 62 whose open bottom end is connected to the precursor feed duct 3.

  It is obvious that devices having structural variations such as the orientation, the position and the number of injection rods 64, the degree of inclination of the vibrating table 60 for example, can be used in an equivalent manner in the present invention. invention.

  When the precursors PR2 are in the form of microtubes of pure silica, they must necessarily be subjected to mechanical pre-treatment of cutting, which is illustrated in FIG. 15, before being heat-treated in the inductive plasma reactor 1 P. The microtubes PR2 have a silica purity level of greater than 99.9%, preferably greater than 99.99% and an outer diameter of between 25 and 1000 m. They are arranged in the form of a sheet of microtubes PR2 on a feed table (not shown).

  The feed table consists of a beam supporting the sheet of microtubes PR2, the beam being itself supported by rollers. A laser cutting device 7 is disposed at the end 72a of the sheet of microtubes PR2. The direction of the laser 7 is perpendicular to that of the microtubes PR2.

  The rotated rollers transmit a translation movement to the laser cutting device 7 to the beam they support. The beam which supports the sheet of microtubes PR2 advances towards the laser cutting device 7 and conveys, in this way, same movement, microtubes PR2 to the laser cutting device 7. The beam also allows to maintain the microtubes PR2 aligned and straight.

  The microtubes PR2 are then cut by a laser 7 into precursors PR2 'in the form of sections which have a length of between 1 and 1.5 times the diameter of the microtubes PR2. The rollers are set to advance the sheet of microtubes PR2, between each cutting step, the length of the precursors PR2 '.

  The cutting by the laser 7 is performed by a vertical scan of the laser 7 between the upper generatrix 72 microtubes PR2 (also called contact point) and the lower generatrix 73. The cutting step by the laser 7 leads to the formation precursors PR2 'in the form of sections of length defined as a function of the diameter of microtubes PR2 and each end of which is closed by autogenous welding induced by laser cutting 7.

  Advantageously, before the cutting step, the sheet of microtubes PR2 is frosted at the level of the upper generatrices 72 of the microtubes PR2 (process not shown) chemically (in the presence of a low pH), thermally (partial melting) or mechanically (by abrasion). This grinding step saves part of the large amount of energy consumed by the laser 7 during the priming of the cut at the level of the upper generator 72 of the microtubes PR2.

  Continuous scanning by a flushing gas 71 such as helium or hydrogen from all of the microtubes PR2 is preferably carried out continuously during the laser cutting step 7 of the microtubes PR2. Thus, the closing of the ends of the precursors PR2 'induced by the laser cutting 7 makes it possible to enclose the flushing gas 71 within the precursors PR2'. Thus, this continuous scanning step also makes it possible to control the gas 71 enclosed within the precursors PR2 '. The sweeping can also be performed with a rare gas and in particular argon.

  The precursors PR2 'obtained are collected in a receptacle 74 and whose open, lower end directs the precursors PR2' to a distributor (not shown) which feeds the precursor feed duct 3 of the inductive plasma reactor 1 P. It is obvious that laser cutting devices having structural variations such as the orientation and the positioning of the sheet of microtubes PR2 and the laser 7, the direction and the cutting path by the laser 7 for example, can be used equivalently in the present invention.

  The microspheres M derived from the precursors PR1, PR1 'and PR2' can be used as such in particular as a lightening filler allowing, when mixed with a composition (for example concrete), to reduce the final density of this composition. .

  However, the microspheres M can also be joined by assembly in order to facilitate their handling. They can be assembled in plates or tubular sleeves for example. These assemblies can also be reinforced by incorporating textiles such as webs, webs, thick fabrics or webs, fibers, microtubes or silica nanopowders.

  The assembly can be carried out by two different processes: by agglomeration of the microspheres M with an agent agglomerating for example with starch, and by sintering.

  Agglomerating assembly of the microspheres M with an agglomerating agent such as starch (not shown) comprises first of all the step of mixing the microspheres M with an aqueous suspension of starch and / or any other agent. agglomeration which does not deteriorate the properties of microspheres M. This step is preferably preceded by a granulometric separation step in a device such as that illustrated in Figures 6 or 7 except that the hopper 32 distributes in this case microspheres M. Then the aqueous suspension comprising the microspheres M and the hydrated agglomerating agent is concentrated (drained) by filtration in a device similar to that illustrated in FIG. 5 except that the hopper 29 dispenses in this case the aqueous suspension comprising the microspheres M and the hydrated agglomerating agent. The suspension thus concentrated and transferred using a textile filter similar to the textile filter 25 of Figure 5 in a baking mold. The concentrated suspension is preferably broken for example in a vibrating funnel which distributes the concentrated suspension in the baking mold. Finally, once in the baking mold, the concentrated suspension is preferably compacted and then heat treated between 300 and 400 C in the case where starch is used as agglomerating agent. The heat treatment can be carried out in a conventional thermal oven, or with cooking plates or presses placed on the surface of the concentrated suspension or by passing the concentrated suspension under heating rollers. In the case where starch is used as an agglomerating agent, cooking enables the starch (starch) to gel, which in gelled form makes the microspheres M adhere to one another. The cooking then allows the structure to dry and thus freeze, solidify and stabilize it.

  This method of agglomerating the microspheres M with an agglomerating agent can be carried out continuously.

  The assembly by sintering consists in adhering the microspheres M to each other by superficial softening of their wall at temperatures up to 1650 C. The adhesion is performed at the point of contact of the microspheres M. This adhesion is amplified by the mixture M microspheres with silica nanopowder (as defined above) prior to the heating step up to 1650 C. At this temperature, the silica nanopowder melts and welds the microspheres M to each other at their point of contact.

  The sintering process can be continuous or discontinuous.

  The device used to carry out a discontinuous sintering is illustrated in FIG. 12 for the pre-sintering step comprising the steps of mixing the microspheres M with the silica N nanopowder and the microwave application and in FIG. the sintering step.

  In FIG. 12, the mixing device 80 is of the same type as the mixing device 40 illustrated in FIG. 8. It comprises hoppers 81 and 82 respectively distributing microspheres M and silica nanopowder N. A receptacle 80b collects the composition M 'resulting from the mixing of the microspheres M with silica nanopowder N. A sintering preparation cell 87 consists of a chamber at the bottom of which is deposited a tray 84 preferably made of Teflon and comprising oxalic acid 85. The sintering preparation cell 87 is traversed at its upper wall of the lower part of the receptacle 80b. It finally comprises a removable loading basket 83 disposed above the tray 84 (by holding means not shown).

  The removable loading basket 83 preferably Teflon has a base 83b screened orifices advantageously of the order of 1 mm spaced every 5 mm and covered over its entire surface with a fabric 83a.

  The sintering preparation cell 87 also includes a microwave generator 86.

  During this sintering preparation step, the receptacle 80b deposits the composition M 'in the removable loading basket 83. The textile filter 83a makes it possible to prevent the composition M' from penetrating into the orifices of the base 83b. It finally allows the penetration of oxalic acid vapors 85 in the composition M 'which promotes the adhesion of the silica particles during the treatment with microwaves. Once deposited in the removable loading basket 83, the composition M 'is preferably compacted and then microwaves are applied in the chamber 87. After application of the microwaves, the composition M' thus heated is summarily hardened and assembled. Finally, the textile filter 83a makes it possible to transfer the composition M 'to a teflon baking mold 89 (or to sintered silica and silicon carbide) so that the composition M' is subjected to a sintering cycle in an oven thermal 88 as shown in Figure 13.

  The thermal furnace 88 consists of three chambers: a preheating furnace 88a, a residence furnace 88b and a cooling furnace 88c which are separated by zone locks 88d. Zone locks 88d are preferably constituted by removable insulating walls but may also consist of distances between the chambers large enough for the different chambers to retain their own thermal cycles. The preheating oven 88a, the living oven 88b and the cooling furnace 88c are equipped with burners (not shown) gas or electric. The preheating furnace 88a may also be equipped with a microwave generator (not shown) for the same purpose as that previously described in FIG. 12. The various furnaces 88a, 88b and 88c may also be equipped with control systems autonomous (not shown). Preferably, each oven 88a, 88b or 88c has a volume of between 4 and 6 m3, and can reach a temperature up to 1700 C.

  The composition M 'transferred into a baking mold 89 is first preferably packed and then successively placed in the preheating oven 88a, then the living oven 88b and the cooling furnace 88c.

  The duration of a complete sintering cycle is between 8 and 10 hours. The duration of a cooking cycle is as follows: 120 minutes in the preheating furnace 88a (with a gradual increase in temperature from 20 ° C. to 1000 ° C. in a first step and from 1000 ° C. to 1200 ° C. in a second step) ; 120 minutes in the residence oven 88b (with constant residence temperature in a range from 1200 C to 1650 C) and 300 minutes in the cooling furnace 88c (with a progressive decrease in residence temperature at 20 C). The gradual decrease in temperature makes it possible to avoid a sudden thermal shrinkage of the sintered composition M 'which could damage the structure of the sintered composition M'.

  In order to increase the productivity of the process, it is preferable to provide the thermal furnace 88 with a second cooling furnace 88c because the cooling cycle is relatively long in comparison with the preheating and residence cycle.

  It is obvious that any other heat treatment device, for example, cooking plates or presses placed on the surface of the composition M ', heating rollers, or systems for turning the Misfed composition to heat both sides of the composition. the sintered composition M 'may be used in an equivalent manner in the context of the present invention.

  The device used to carry out continuous sintering is illustrated in FIG.

  In FIG. 14, a hopper 91 is disposed above a first end 90a of a conveyor belt 90. The progress of the conveyor belt 90, the direction of which is illustrated by the reference arrow 90d, is induced by the rotation of the rolls 90b. The treadmill has a second end 90c. Between the ends 90a and 90c, are successively arranged a heating device 93 disposed above the treadmill 90, consisting of an electrical resistance 93a and a reflector 93b and a cooling tunnel 94 through which the treadmill 90 passes. A device creating a local depression 92 is provided at the heater 93 and juxtaposed to the underside of the treadmill 90. The treadmill 90 is preferably made of a silica sheet.

  The continuous sintering is preferably preceded by a step of mixing the microspheres M with silica nanopowder N prior to the heating step as described for FIG. 12. The composition M 'resulting from this mixture is fed continuously to the end 90a of the treadmill 90. The advancement of the treadmill 90 conveys the composition M 'below the electrical resistance 93a and the reflector 93b. The electrical resistance 93a makes it possible to heat the composition M 'so as to adhere the microspheres M to each other by partial melting of the surface of their wall and of the silica nanopowder. The reflector 93b concentrates the thermal radiation from the electrical resistance 93a on the composition M '. The device 92 creating a local depression 92b temporarily brings the balls together, which improves the heat exchange and thus promote the partial melting of their walls with each other or with the silica nanopowder. The device 92 creating a local depression 92b has in its upper part crushed silica 92a which locally supports the treadmill 90 and which prevents the deformation of the treadmill 90 generated by the applied local depression 92b. The composition M 'thus sintered after passing through the electrical resistance 93a, forms a continuous plate 96 which passes into the cooling tunnel whose action is to progressively cool the continuous plate 96 in order to avoid a sudden thermal contraction which could damage the structure of the sintered composition M '. At the outlet of the cooling tunnel, the continuous plate 96 is cut by a conventional cutting device 95 (of guillotine or saw type) which delivers sections of plate 97 of composition M 'sintered. Subsequently, the plates 97 may be inverted by plate reversers or superimposed on other plates 97 to undergo further heat treatment by passing under the resistor 93.

  The advantage of the sintering assembly is that the parts thus assembled can withstand operating temperatures of up to 1650 C or even 2400 C for wear parts.

  The compactness of the assembled microspheres can vary according to the different parameters of the process described above (for example the temperature), and has an influence on the physicochemical properties of the assembled microspheres, in particular in terms of thermal insulation, density and mechanical properties (for example, greater compactness decreases thermal insulation).

  The microspheres M originating from the PR2 'precursors may also be used in a particular application of a hydrogen or helium reservoir 77. The device making it possible to convert the microspheres M into a hydrogen or helium reservoir 77 is illustrated in FIG. 16.

  Firstly, the microspheres M are deposited in an induction furnace 75 which consists of a sealed chamber of cylindrical shape whose upper and lower faces form two sealed flange openings (not shown) after opening the upper flange. from the oven. The upper flange of the induction furnace 75 is then closed.

  Then the induction furnace 75 is purged with helium (step not shown) by a continuous helium sweep between the gas inlet valve 75a and the gas evacuation valve 75b of the induction reactor 75.

  This step makes it possible to replace the gaseous environment of the microspheres M constituted by air by a chemically neutral environment consisting of helium. Then the gas evacuation valve 75b is closed and a gas such as helium or hydrogen 77 is injected into the induction furnace 75 under a pressure of 10 'to 2x 10' Pa for a few tens of minutes for a volume of the order of 100 liters of microspheres M. Then the induction furnace 75 is heated by induction, thanks to the inductor 76, so as to provide a temperature greater than or equal to 800 C. Finally, the induction furnace 75 is cooled under the same pressure conditions as those of the heating step. Finally, the microspheres are evacuated from the induction furnace 75 by the opening of the lower flange. When the gas stored is hydrogen, the various steps of the storage method are performed under a controlled atmosphere. Alternatively, other heating means than induction heating may be used.

  When heated to more than 800 ° C., the silica contained in the wall of the microspheres M becomes porous to small space-space gases such as helium and hydrogen. During the induction furnace heating step 75, helium or hydrogen 77 thus enters the microspheres M. Then the cooling induces a decrease in the porosity of the silica with respect to the helium or hydrogen 77 which is enclosed in the microspheres M. The microspheres M thus treated thus contain, at ambient temperature, helium or hydrogen under high pressure (greater than 10 Pa) and can be used as helium or hydrogen reservoirs 77 capable of releasing helium or hydrogen 77 by simple heating at 850 ° C. at atmospheric pressure.

  The advantage of this particular application of the microspheres M lies in the fact that, thanks to the very high silica purity level, the porosity of the microspheres M is controllable and gas such as helium or hydrogen can be stored in a controlled manner. Thus, the use of this type of microspheres M is not dangerous because the microspheres M contain a very small amount of gas stored by microsphere, and they become permeable to gases only from In addition, this new form of M microsphere gas storage is convenient because it allows the user to safely handle small volumes of gas or precisely determinable gas volumes.

  Finally, the silica microspheres can be used for flocking primer concrete, plaster or mechanical alloy especially when the flocked surface is subjected, in use, to temperatures above 1600 C. The flocking process is carried out in a conventional manner (process not shown) and comprises a first step of mixing the silica microspheres with silica nanopowder and silica fibers in a conventional mixer, for example of the type illustrated in FIG. 8 and a second step of projecting the composition resulting from the mixing of the first step by a thermal lance, for example an inductive plasma or electric arc thermal lance. Such flocked materials have good fire resistance and can be used in the field of building, aerospace or shipbuilding.

  Structures including silica microspheres can be effective thermal insulation materials even in the presence of very high temperatures of use when the insulation material is silica sintered microspheres.

  Although the invention has been described in connection with a particular embodiment, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

2875494 30

Claims (25)

  1. Silica microsphere (M) having an outside diameter of between 50 and 125 μm, preferably between 60 and 90 μm, a wall thickness greater than 1 μm, preferably between 1 and 3 μm and a density of between 0 , 3 and 0.7g / cm3.   2. Microsphere of silica (M) according to claim 1, comprising more than 95% by weight of silica relative to the total mass of the silica microsphere (M), preferably more than 99% by weight of silica relative to the total mass of the silica microsphere (M).   3. Process for the production of silica microspheres (M) according to claim 1 or 2, characterized in that it comprises at least one step of injecting at least one precursor of the silica microspheres (MS, PR1, PR1 ', PR2') within an inductive plasma (P), said inductive plasma (P) being preferentially doped with a hydrocarbon such as propane or methane.   4. Method according to claim 3, characterized in that it further comprises a silicon halide injection step (H).   5. Method according to claim 3 or 4, characterized in that it further comprises a step of pretreatment of the precursor of the silica microspheres (PR1, PR2) prior to the step of injecting said precursor (PR1 ', PR2' ) within an inductive plasma (P).   6. Method according to one of claims 3 to 5, characterized in that the precursor of silica microspheres (PR1) is a silica glass powder or quartz particle size less than 51tm, preferably less than 21.tm.   7. Process according to the combination of claims 5 and 6, characterized in that the step of pretreating the precursor of the silica microspheres (PR1) comprises mixing the precursor of the silica microspheres (PRI) in water (W) with at least one additive (AD) which is an expansion agent and / or a binding agent and / or a fluxing agent, the precursor composition (PR1 ') resulting from the mixture being then injected into an inductive plasma ( P).   8. Process according to claim 7, characterized in that the precursor composition (PR1 ') comprises at least 50% by weight of silica microsphere precursor (PRI) relative to the total mass of said precursor composition (PR1'). ), preferably at least 60% by weight of silica microsphere precursor (PR1) relative to the total mass of the precursor composition (PR1 ').   9. The method of claim 7 or 8, characterized in that said precursor composition (PR1 ') comprises precursor silica microspheres (PR1) and seawater.   10. Method according to one of claims 7 to 9, characterized in that the precursor composition (PR1 ') is further concentrated by filtration before being injected into an inductive plasma (P).   11. The method of claim 10, characterized in that the filtration step further comprises a granulometric separation step before injection.   12. The method of claim 7, characterized in that droplets of said precursor composition (PR1 ') are coated with synthetic silica powder (S) before being injected into an inductive plasma (P).   13. The method of claim 12, characterized in that the step of coating with synthetic silica powder (S) is carried out by spraying the precursor composition (PR1 ') on a vibrating table (60) supporting a anti-wetting agent (AM) and said synthetic silica powder (S), the anti-wetting agent (AM) being preferentially constituted by material of plant origin, in particular lycopodium spores.   14. The method of claim 5, characterized in that the precursor of silica microspheres (PR1 ') is in the form of silica microtubes (PR2).   15. The method of claim 14, characterized in that the silica purity level of the silica microtubes (PR2) is greater than 99.9% by weight.   16. Method according to one of claims 14 or 15, characterized in that the pretreatment step of the silica microsphere precursor (PR2) comprises a step of cutting the silica microtubes (PR2) laser (7).   17. The method of claim 16, characterized in that the step of cutting the silica microtubes (PR2) further comprises a simultaneous scanning step with a flushing gas (71).   18. Assembly of the silica microspheres (M) according to claim 1 or 2, by agglomeration of the microspheres (M) with an agglomerating agent.   19. The assembly of the silica microspheres (M) according to claim 1 or 2, by batch sintering which comprises the steps of: • mixing silica microspheres (M) and silica nanopowder (N); composition (M ') resulting from this mixture in 1 o a mold (89), é Preheating the composition (M') by application of microwaves preferentially in the presence of oxalic acid vapors (85) and heating the composition ( M ') preheated in a thermal oven (88).   20. The assembly of the silica microspheres (M) according to claim 1 or 2, by continuous sintering which comprises the steps of: E Mixing silica microspheres (M) and silica nanopowder (N), composition (M ') resulting from this mixture in the presence of an electrical resistance (93) by depositing said composition (M') continuously on a treadmill (90) and temporarily immobilizing said composition (M ') at the level of the electrical resistance (93).   21. Use of silica microspheres (M) according to claim 1 or 2, in the flocking of surfaces such as concrete, plaster or a metal alloy.   22. Use of silica microspheres (M) according to claim 21, characterized in that said flocking comprises a first step of mixing the silica microspheres (M) with silica nanopowder (N) and silica fibers and a second projection step of the composition resulting from the mixing of the first step by thermal lance.   23. Use of silica microspheres (M) according to claim 1 or 2 as a thermal insulation material.   24. Use of silica microspheres (M) from the process according to one of claims 14 to 17 for the storage of gas (77).   25. The method of storing gas according to claim 24, characterized in that it comprises a step of heating silica microspheres (M) at a temperature greater than or equal to 800 C in the presence of helium or hydrogen (77). ) at a pressure greater than or equal to 10 Pa.